Houdini Fly (Cacoxenus indagator)
A new “bad guy” in the area, the Houdini fly sneaks into the mason bee nest when the female is out gathering nectar and pollen. After the Houdini fly lays her eggs, she leaves and her larva use the developing mason bee as a host.
The best way to eliminate this fly is to purchase mason bee cocoons that have been cleaned and are not still in nesting material. If you do have cocoons in straws and nesting material, plan to “harvest” them in the fall to remove any parasites (especially the Houdini fly). There are classes in October to help with this process. Check with WSU Extension Office.